CYP2A6 and chronic obstructive pulmonary disease: Genome-wide association studies (GWAS) have identified genes with single-nucleotide polymorphisms (SNPs) associated with COPD such as CHRNA3/CHRNA5 [47], HHIP [48], FAM13A [49], and CYP2A6 [50]; these SNP susceptibility loci correlate with different phenotypes or disease severity of COPD.